1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a hybrid automatic repeat request (HARQ) feedback and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Different from the LTE/LTE-A system with frequency-division duplexing (FDD), transmission directions of subframes of a frequency band in the LTE/LTE-A system with time-division duplexing (TDD) may be different. That is, the subframes are divided into uplink (UL) subframes, downlink (DL) subframes and special subframes according to the UL/DL configuration specified in the 3GPP standard.
Please refer to FIG. 1 which is a table 10 of the UL/DL configuration with corresponding factors m for numbers of physical HARQ indicator channel (PHICH) groups, wherein the number of PHICH groups for a specific subframe is determined as m·NPHICHgroup. In FIG. 1, 7 UL/DL configurations are shown, wherein each of the UL/DL configurations indicating a set of transmission directions for 10 subframes, respectively. In detail, “U” means that the subframe is a UL subframe where UL data is transmitted, and “D” means that the subframe is a DL subframe where DL data is transmitted. “S” means that the subframe is a special subframe where control information is transmitted, and the special subframe can also be simply seen as the DL subframe in the present invention. Since a HARQ feedback corresponding to a UL transmission can only be transmitted in the DL subframes, the number of PHICH groups M·NPHICHgroup is only nonzero for the DL subframes and the special subframes as shown in FIG. 1.
Furthermore, a UL/DL configuration of a legacy UE can be changed according to System Information Block Type 1 (SIB1) transmitted by an eNB, e.g., from the UL/DL configuration 1 to the UL/DL configuration 3. A minimum periodicity of transmitting the SIB1 is usually large (e.g., 640 ms), and the legacy UE can only change the UL/DL configuration with the periodicity of 640 ms. The semi-statics allocation cannot match fast varying traffic characteristics and environments, and there is space for improving system performance. Thus, changing the UL/DL configuration with a lower periodicity (e.g., lower than 640 ms) is considered.
In general, the legacy UE is configured with a UL/DL configuration according to the SIB1, and this UL/DL configuration is also known by an advanced UE which is configured with an additional UL/DL configuration. The additional configuration is a real configuration operated by the eNB, and the eNB provides services (i.e., performs transmissions and/or receptions) to both the legacy UE and the advanced UE according to the additional configuration. For backward compatibility, the eNB may schedule the legacy UE to avoid accessing a subframe that is configured with different transmission directions due to the UL/DL configuration and the real UL/DL configuration. That is, when the advanced UE which can change the UL/DL configuration fast operates in the LTE/LTE-A system, it may happen that the advanced UE intends to receive a HARQ feedback in a subframe (i.e., DL subframe) while the subframe is the UL subframe for the legacy UE. For example, the advanced UE and the legacy UE are configured with the UL/DL configurations 2 and 3, respectively, and the advanced UE intends to receive the HARQ feedback in the subframe 3. If the eNB simply transmits HARQ feedbacks of the UEs, the advanced UE can receive its HARQ feedback while the legacy UE lose its HARQ feedback without any UL transmission.
Alternatively, the advanced UE may intend to receive the HARQ feedback in a subframe (i.e., DL subframe) which is also the DL subframe for the legacy UE. However, the number of PHICH groups m·NPHICHgroup for the advanced UE is greater than the number of PHICH groups m·NPHICHgroup for the legacy UE due to different UL/DL configurations of the UEs, wherein NPHICHgroup is a basic number of the PHICH groups and is usually broadcasted in master information block (MIB) via a physical broadcast channel (PBCH). For example, the advanced UE and the legacy UE are configured with the UL/DL configurations 0 and 6, respectively, and the advanced UE intends to receive the HARQ feedback in the subframe 5. The HARQ feedback can not be arranged in the PHICH with 2NPHICHgroup PHICH groups corresponding to the UL/DL configuration 0 of the advanced UE, since the legacy UE detects the PHICH according to a detecting rule for NPHICHgroup PHICH groups and operation of the legacy UE should not be affected. The above situations usually occur when the advanced UE changes its UL/DL configuration while the legacy UE does not.
Thus, how to receive the HARQ feedback when the conflict occurs due to different UL/DL configurations of the UEs is an important topic to be discussed.